Free-space optical hybrid

ABSTRACT

An exemplary optical hybrid includes a 50/50 un-polarized beam splitter, a folding prism, a beam shifter, a spacer and a phase shifter such that from an input S-beam (signal) and an L-beam (reference), four outputs, S+L, S−L, S+jL, S−jL, are produced. The phase difference of S and L at the four output ports is phi_0, phi_0+90-degrees, phi_0+180-degree, phi_0+270-degree, where the phase difference between S and L at the S+L port is an arbitrary number phi_0 and the phase difference between S and L at the other output ports, S−L, S+jL, S−jL, is 90, 180 and 270, respectively, shifted from phi_0.

This application claims priority to Provisional Patent Application Ser.No. 60/786,630, titled “Free-Space Optical Hybrid” filed Mar. 27, 2006,incorporated herein by reference. This application claims priority toProvisional Patent Application Ser. No. 60/899,579, titled “Thin FilmCoated Optical Hybrid” filed Feb. 2, 2007, incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to coherent detection, and morespecifically, it relates to a low cost, compact, andtemperature-insensitive optical hybrid.

2. Description of Related Art

Since the late 1990s, the transport capacities of ultra-long haul andlong-haul fiber-optic communication systems have been significantlyincreased by the introduction of the erbium-doped fibre amplifier(EDFA), dense wavelength division multiplexing (DWDM), dispersioncompensation, and forward error correction (FEC) technologies. Forfiber-optic communication systems utilizing such technologies, theuniversal on/off-keying (OOK) modulation format in conjunction withdirect detection methods have been sufficient to address data rates upto 10 Gb/s per channel.

In order to economically extend the reach and data capacity beyond suchlegacy systems and into next-generation networks, several technologicaladvancements must take place, including but not limited to, 1) adoptionof a differential phase-shift keying (DPSK) modulation format, asopposed to OOK; 2) developments in optical coherent detection; and 3)progress in adaptive electrical equalization technology. In combination,these technologies will boost a signal's robustness and spectralefficiency against noise and transmission impairments.

Such crucial strides in optical signal technology are no longertheoretical possibilities but are feasible solutions in present-dayoptical networking technology. The path for an optical coherent systemhas already been paved by 1) the deployment of DPSK modulated systems byTier-1 network providers; and 2) the increased computational capacityand speed of electronic DSP circuits in receivers, which provides anefficient adaptive electrical equalization solution to the costly anddifficult optical phase-lock loop. These advances coupled with acommercially feasible optical hybrid solution would likely give pause toTier-1 providers and carriers to reassess their earlier rationales fornot adopting and implementing an optical coherent detection scheme.Perhaps with such advances, optical networks will begin to realize thebenefits already recognized in microwave and RF transmission systems forextending capacity and repeaterless transmission distances throughcoherent detection.

The commercial feasibility of a coherent system for optical signaltransmission was first investigated around 1990 as a means to improve areceiver's sensitivity. In contrast to existing optical direct-detectionsystem technology, an optical coherent detection scheme would detect notonly an optical signal's amplitude but phase and polarization as well.With an optical coherent detection system's increased detectioncapability and spectral efficiency, more data can be transmitted withinthe same optical bandwidth. More over, because coherent detection allowsan optical signal's phase and polarization to be detected and thereforemeasured and processed, transmission impairments which previouslypresented challenges to accurate data reception, can, in theory, bemitigated electronically when an optical signal is converted into theelectronic domain. However, the technology never gained commercialtraction because the implementation and benefits of an optical coherentsystem could not be realized by existing systems and technologies.

Implementing a coherent detection system in optical networks requires 1)a method to stabilize frequency difference between a transmitter andreceiver within close tolerances; 2) the capability to minimize ormitigate frequency chirp or other signal inhibiting noise; and 3) anavailability of an “optical mixer” to properly combine the signal andthe local amplifying light source or local oscillator (LO). Thesetechnologies were not available in the 1990s. A further setback to theadoption and commercialization of an optical coherent system was theintroduction of the EDFA, an alternative low cost solution to thesensitivity issue.

Notwithstanding the myriad challenges, an optical coherent system (alsoreferred to as “Coherent Light Wave”) remains a holy grail of sorts tothe optical community because of its advantages over traditionaldetection technologies. Coherent Light Wave provides an increase ofreceiver sensitivity by 15 to 20 dB compared to incoherent systems,therefore, permitting longer transmission distances (up to an additional100 km near 1.55 μm in fiber). This enhancement is particularlysignificant for space based laser communications where a fiber-basedsolution similar to the EDFA is not available. It is compatible withcomplex modulation formats such as DPSK or DQPSK. Concurrent detectionof a light signal's amplitude, phase and polarization allow moredetailed information to be conveyed and extracted, thereby increasingtolerance to network impairments, such as chromatic dispersion, andimproving system performance. Better rejection of interference fromadjacent channels in DWDM systems allows more channels to be packedwithin the transmission band. Linear transformation of a receivedoptical signal to an electrical signal can then be analyzed using modernDSP technology and it is suitable for secured communications.

There is a growing economic and technical rationale for adoption of acoherent optical system now. Six-port hybrid devices have been used formicrowave and millimetre-wave detection systems since the mid-1990s andare a key component for coherent receivers. In principle, the six-portdevice consists of linear dividers and combiners interconnected in sucha way that four different vectorial additions of a reference signal (LO)and the signal to be detected are obtained. The levels of the fouroutput signals are detected by balanced receivers. By applying suitablebaseband signal processing algorithms, the amplitude and phase of theunknown signal can be determined.

For optical coherent detection, a six-port 90° optical hybrid should mixthe incoming signal with the four quadratural states associated with thereference signal in the complex-field space. The optical hybrid shouldthen deliver the four light signals to two pairs of balanced detectors.Let S(t) and R denote the two inputs to the optical hybrid and

${{S(t)} + {R\;{\exp\left\lbrack {j\left( {\frac{\pi}{2}n} \right)} \right\rbrack}}},$with n=0, 1, 2 and 3, represent the four outputs from it Using the PSKmodulation and phase-diversity homodyne receiver as an illustration, onecan write the following expression for the signal power to be receivedby the four detectors:

${{P_{n}(t)} \propto {P_{S} + P_{R} + {2\sqrt{P_{S}P_{R}}{\cos\left\lbrack {{\theta_{S}(t)} + {\theta_{C}(t)} - {\frac{\pi}{2}n}} \right\rbrack}}}},{n = 0},{{\ldots\mspace{11mu} 3};}$where P_(S) and P_(R) are the signal and reference power, respectively,θ_(S)(t) the signal phase modulation, and θ_(C)(t) the carrier phaserelative to the LO phase. With proper subtractions, the twophotocurrents fed to the TIA's can be expressed asI _(BD1)∝√{square root over (P _(S) P _(R))} cos[θ_(S)(t)+θ_(C)(t)];I _(BD2)∝√{square root over (P _(S) P _(R))} sin[θ_(S)(t)+θ_(C)(t)];encompassing the amplitude and phase information of the optical signal.Accordingly, the average electrical signal power is amplified by afactor of 4P_(R)/P_(S). Following this linear transformation the signalsare electronically filtered, amplified, digitized and then processed.Compared to a two-port optical hybrid, the additional two outputs haveeliminated the intensity fluctuation from the reference source (LO).

An optical coherent receiver requires that the polarization state of thesignal and reference beam be the same. This is not a gating item asvarious schemes or equipment are available to decompose and control thepolarization state of the beams before they enter the optical hybrid.Further, certain polarization controllers can be used to provideadditional security functionality for optical coherent systems,preventing third parties from tapping information or data streams byimplementing polarization scrambling and coding techniques.

For laboratory purposes, a 90° optical hybrid has traditionally beenconstructed using two 50/50-beam splitters and two beam combiners, plusone 90° phase shifter. These optical hybrids can be implemented usingall-fiber or planar waveguide technologies; however, both methods havetheir respective drawbacks. Both technologies require sophisticatedtemperature control circuits to sustain precise optical path-lengthdifference in order to maintain an accurate optical phase at theoutputs. In addition, fiber-based devices are inherently bulky and areunstable with respect to mechanical shock and vibration; whereas,waveguide-based products suffer from high insertion loss, highpolarization dependence and manufacturing yield issues. Waveguide-basedproducts are also not flexible for customization and require substantialcapital resources to set up.

Accordingly, a low-cost, temperature insensitive and vibration/shockresistant optical hybrid and method of operating same is desirable andsuch is provided by the present invention.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a free-space typeinterferometer to achieve the hybrid function.

Another object is to provide a low-cost, temperature insensitive andvibration/shock resistant optical hybrid.

These and other objects will be apparent based on the disclosure herein.

An embodiment of the present optical hybrid is based on the Michelsoninterferometer principle. The Michelson interferometer principle hasbeen proven and tested in free-space bulk-optics and optical componentmanufacturing. Free-space bulk-optics is a mature technology with aproven track record in providing many critical components, such ascirculators, polarization beam combiners, wavelength lockers, dispersioncompensators, interleavers and DPSK demodulators, to the fiber-opticcommunication industry. In addition, bulk-optics based devices have lowinsertion loss and their core optics can be readily coupled tocommercially available fiber collimators.

The fundamental strength of the free-space-interferometer design is thatone can make the optical path lengths of the various interference beamsin the glass approximately the same such that the phase relationshipamong the different outputs—a key performance parameter—will not besensitive to temperature variation, mechanical shock and/or vibration.

Free from any active control, the present optical hybrids are compact(less than 40×40×14 mm), polarization-independent, and thermally stable.The Michelson-interferometer based optical hybrid can also be integratedwith a polarization controller, a required component for an opticalcoherent receiver. By integrating the optical hybrid and polarizationcontroller into one device, a more cost-effective, compact andperformance-enhanced component can be manufactured. In contrast,all-fiber or waveguide based devices cannot realize these same benefitsbecause these technologies cannot be similarly combined.

The technological barriers against adopting and commercialising anoptical coherent system have been overcome. First, stabilizing thefrequency difference between a transmitter and receiver within closetolerances may be accomplished with current advancement in electronicsand tunable laser sources. Secondly, system designers have begunintroducing the PSK modulation format in network systems, therebyeliminating the frequency chirp associated with OOK. And finally, withthe introduction of the present optical hybrids, the final key componentfor mixing the signal and reference beam in the optical domain hasbecome available.

These advancements in key technological fronts should create sufficientimpetus for carriers to re-examine the benefits and economic rationaleof an optical coherent system. With the present optical hybrids, thefull advantages of a coherent detection scheme may be realized: higherreceiver sensitivity, compatibility with PSK modulation, increasedtolerance to transmission impairment, more channels within the availablebandwidth, and secured communications.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a partof the disclosure, illustrate embodiments of the invention and, togetherwith the description, serve to explain the principles of the invention.

FIG. 1A shows a function diagram for a 90-degree optical hybrid.

FIG. 1B conceptually illustrates the operation of a 90-degree opticalhybrid.

FIG. 2 shows an embodiment of the invention including a 50/50un-polarized beam splitter, a folding prism, a beam shifter, a spacerand a phase shifter.

FIG. 3 shows a design that is similar to FIG. 2 without the circulators.

FIG. 4 illustrates a design having symmetrical cavities in the top andright arms.

FIG. 5 shows a design similar to that of FIG. 4, except that the pathlength ACE is lengthened.

FIG. 6A is a top view of a design integrated with a polarization beamsplitter to provide a polarization diversity coherent detection system.

FIG. 6B is a side view of the design of FIG. 6A.

FIG. 7 shows a 2×4 optical hybrid implemented by 3 independent beamsplitters.

FIGS. 8A and 8B are a top view and a side view, respectively, of a 2×8optical hybrid that includes a PBS and a folding prism.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A shows a function diagram for a 90-degree optical hybrid 10. Thisis a Sport device in that it has two input ports 12, 14 and four outputports 16, 18, 20 and 22. As shown in FIG. 1A, a signal light wave 24 isinput into input port 12 and a reference light wave 26 (from a localoscillator) is input into input port 14. From the four outputs ports 16,18, 20 and 22 exits four outputs 28, 30, 32 and 34 respectively whichare the interferences between the two input light beams, with variousrelative phase shift.

FIG. 1B conceptually illustrates the operation of a 90 degree opticalhybrid. It Consists of two 50/50-beam splitters, one each at 40 and 42and two beam combiners, one each at 44 and 46 plus one 90-degree phaseshifter 48. In the practical implementation, this is achieved bywaveguide technology. The size is large and it requires temperaturecontrol to maintain the required optical path length. In addition tothat, typically, the loss is significant and has strong polarizationdependence.

FIG. 2 shows an embodiment of the invention. The components are a 50/50un-polarized beam splitter 50, a folding prism 52, a beam shifter 54, aspacer (or simply a gap) 56 and a phase shifter 58. The gap can includeAR coating on its surfaces and/or a wedge to prevent back reflections.The abeam 60 and the L-beam 62 (which traverse optical circulators 61and 63 respectively) are incident from the left-hand side of beamsplitter 50 and hit beam splitter 50 at positions A and B. Both beamssplit into two parts—a top component and a right component The topreflection optic is folding prism 52, which includes a total-internalreflection oriented surface 64 plus a mirror coating 66. The rightreflection optics is beam-shifter 54, which includes two total internalreflection oriented surfaces 68 and 70.

Input beams 60 and 62 traverse the optics and four outputs, S+L, SL,S+jL, S+jL, exit from the beam splitter at output ports 01, 02, 03 and04 respectively. In the top optical path, there is an air or gas cavity.The index of refraction may be adjusted within this cavity to perform asa phase shifter. In one embodiment, the index of a gas is adjusted. Inanother embodiment, the cavity contains at least one transparent slab.The illustrated embodiment shows two transparent slabs 58 and 59. Theslabs, typically made by glass material, are used as phase shifters. Inpractical operation, only one slab is needed to perform the phaseshifting function, and the other slab is provided to equalize the pathlengths. By tuning the angle of the slab (or changing the index of agas), one can adjust the optical phase of the light beam. FIG 3 shows asimilar design, with like components accordingly numbered, but withoutusing the circulators of FIG. 2. In this embodiment, the incident beamhas to be tilted slightly.

The phase difference of S and L at the four output ports is phi_(—)0,phi_(—)0+90-degrees, phi_(—)0+180-degree, phi_(—)0+270-degree. In otherwords, the phase difference between S and L at S+L port can be anarbitrary number phi_(—)0. Once this is determined, the phase differencebetween S and L at other output ports are 90, 180, 270 shifted fromphi_(—)0.

In the embodiments shown in FIGS. 2 and 3, the beam splitter is coatedwith a non-absorption material. Under such a condition, the relativephase shift between O1 and O3 is 180 degree, as a result of energyconservation. This is the same reason that the relative phase shiftbetween O2 and O4 is 180 degrees. Based on this, all that must be doneto set the phase difference between S and L at other output ports to be90, 180, 270 shifted from phi_(—)0 is to adjust the relative phase shiftbetween O1 and O2 such that it meets the requirement of 90 degrees.

In both the top and right optical paths of FIGS. 2 and 3, the light beamis totally internally reflected twice before interference occurs. Thiswill make the phase difference of the interference beams to beindependent of the polarization. It has assumed that the beam splitteritself does not introduce any polarization dependent phase. This can beimplemented by coating the beam splitter symmetrically. (See U.S. Pat.No. 6,587,204, incorporated herein by reference). In this design, thereflection optics on the top path is a folding prism, acting as amirror, and that of the right path is a beam shifter. This is the trickused here to bring together the two spatially separated incident beams,S and L. If the polarization effect is not important, the top foldingprism can be replaced by a flat mirror. Ideally, one phase shiftershould be enough to adjust the phase. The second one is mainly toprovide optical path symmetry. It makes the path of both beams on thetop optical path to be symmetrical. As discussed below, to make theoptical paths of the top and right more symmetrical, one can add acavity between the beam splitter and the beam-shifter.

FIG. 4 illustrates a design having symmetric cavities in the top andright arms. A fine phase adjustment is finally achieved by inserting athin glass slab as discussed above to provide angular tuning. In thisdesign, the phase matching condition is attainable (±¼ wavelength) whenX1−Y1=X2−Y2, where the optical paths are ACE=X1, BDF=X2, AGHB=Y1 andBHGA=Y2. The cavity in the right arm is provided by inserting spacer 72.Note also that with the use of the air cavity in both the top and rightarm, it is easier to make the device stable over different temperatures.

The design in FIG. 5 is similar to the design of FIG. 4, except that thepath length ACE is lengthened with the addition of optical element 80,which is a piece of material having an appropriate index of refractionto the wavelength of use and further includes a reflective surface 82.Thus, ACE˜BDF. Here, the phase match condition is achieved whenX1˜X2˜Y1=Y2, and X1−Y1=X2−Y2 (±¼ wavelength) wherein ACE=X1. BDF=X2,AGHB=Y1 and BHGA=Y2.

FIG. 6A shows a top view and 6B shows a side view of a design integratedwith a polarization beam splitter 90 (not visible in FIG. 6A) andreflecting prism 92 to provide a polarization diversity coherentdetection system. The PBS on the left side of the beam splitterseparates the two orthogonal polarizations X and Y of S and L intoS_(x), S_(y), L_(x) and L_(y).

FIG. 7 shows a 2×4 optical hybrid implemented by 3 independent beamsplitters. The 2 inputs are S (signal) and L (local oscillator). The 4outputs are S+L, S+jL, S−L, S−jL. The device includes an unpolarizedbeam splitter (UPB) 100. The upper and right arms are approximatelysymmetrical. The upper arm includes an optical element 102 connected onone of its sides to the upper face of UPB 100 and connected on the otherside to an UPB 104. A folding prism 106 is connected to the right faceof UPB 104. In this embodiment, a slab 108 of optically transmissivematerial is located in a space between the folding prism 106 and the UPB100. The right arm is symmetrical with the upper arm. The right armincludes an optical element 110 connected on one of its sides to theright face of UPB 100 and connected on the other side to an UPB 112. Afolding prism 114 is connected to the upper face of UPB 112. In thisembodiment, a slab 116 of optically transmissive material is located ina space between the folding prism 114 and the UPB 100. In operation, toproduce the desired relative phase between the S and L beams at eachoutput port, the slab 108 is adjusted; however, as discussed in theembodiments of FIGS. 2 and 3, a cavity can be formed by enclosing thespace between the UPB 100 and the folding prisms 106 and/or 114 and theindex of refraction of a gas can be altered to adjust the phase.

FIG. 8A is a top view and 8B is a side-view a 2×8 optical hybrid thatintegrates a PBS 120 (not visible in FIG. 8A) and folding prism 122 intothe module. The 2 inputs are S and L and the 8 outputs are Sx+Lx,Sx+jLx, Sx−Lx, Sx−jLx, Sy+Ly, Sy+jLy, Sy−Ly, Sy−jLy. With thisarrangement, the splitting ratio of the beam splitter can be differentfrom the beam combiners. This provides the freedom to adjust therelative amplitudes at the output ports. Dual fiber collimators are notrequired.

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The foregoing applications, and all documents cited therein or duringtheir prosecution (“appln cited documents”) and all documents cited orreferenced in the appln cited documents, and all documents cited orreferenced herein (“herein cited documents”), and all documents cited orreferenced in herein cited documents, together with any manufacturer'sinstructions, descriptions, product specifications, and product sheetsfor any products mentioned herein or in any document incorporated byreference herein, are hereby incorporated herein by reference, and maybe employed in the practice of the invention.

The foregoing description of the invention has been presented forpurposes of illustration and description and is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. The embodiments disclosed were meant only to explain theprinciples of the invention and its practical application to therebyenable others skilled in the art to best use the invention in variousembodiments and with various modifications suited to the particular usecontemplated. The scope of the invention is to be defined by thefollowing claims.

1. An optical hybrid, comprising: means for splitting a signal lightbeam (S) into a reflected signal light beam (RS) and a transmittedsignal light beam (TS); means for splitting a reference light beam (L)into a reflected light beam (RL) and a transmitted light beam (TL); andmeans for combining pairs of said RS, TS, RL and TL to produce aplurality of combined output beams comprising an S+L beam, an S+jL beam,an S−L beam and an S jL beam, wherein the phase difference between saidS light beam and said L light beam in said S+jL beam, said S−L beam andsaid S−jL beam, with respect to the phase difference between said Slight beam and said L light beam in said S+L beam, is about 90 degrees,180 degrees and 270 degrees, respectively; and wherein said means forsplitting a signal light beam and said means for splitting a referencelight beam together comprise a first beam splitter comprising a firstface, a second face, a third face, a fourth face and an internalreflecting/transmitting coating.
 2. The optical hybrid of claim 1,wherein said means for combining comprises: a reflecting surface fixedlyseparated by a gap from said second face; means for adjusting the indexof refraction N within said gap; and a beam shifter fixedly connected tosaid third face.
 3. The optical hybrid of claim 1, wherein said beamsplitter is selected from a group consisting of an unpolarized beansplitter and a beam splitter designed for only one polarization.
 4. Theoptical hybrid of claim 3, wherein said unpolarized beam splittercomprises a cube.
 5. The optical hybrid of claim 2, wherein said meansfor adjusting the index of refraction N comprises means for adjustingthe index of refraction of a gas within said gap.
 6. The optical hybridof claim 2, wherein said means for adjusting the index of refraction Ncomprises at least one optical flat positioned within said gap.
 7. Theoptical hybrid of claim 1, further comprising a first optical circulatorpositioned to receive and operatively align said S beam into said firstface, wherein said S+L team will exit said first optical circulator,said optical hybrid further comprising a second optical circulatorpositioned to receive and operatively align said L beam into said firstface, wherein said S+jL beam will exit said second optical circulator.8. The optical hybrid of claim 2, further comprising a second spacerfixedly attached to said third face, wherein said beam shifter isfixedly connected by said second spacer to said third face.
 9. Theoptical hybrid of claim 8, wherein said third face, said spacer and saidbeam shifter form a second gap.
 10. The optical hybrid of claim 9,further comprising means for adjusting the optical path length throughsaid second gap.
 11. The optical hybrid of claim 10, wherein said meansfor adjusting the index of refraction N comprises means for adjustingthe index of refraction of a gas within said second gap.
 12. The opticalhybrid of claim 10, wherein said means for adjusting the index ofrefraction N comprises at least one second optical flat positionedwithin said second gap.
 13. The optical hybrid of claim 8, furthercomprising an optical path addition fixedly attached to said foldingprism.
 14. The optical hybrid of claim 1, further comprising means forseparating the polarizations of said S beam and said L beam prior totheir being operatively aligned into said first face.
 15. The opticalhybrid of claim 14, wherein said means for separating the polarizationscomprises a polarizing beam splitter (PBS) configured to transmit onepolarization component (Y) into said first face and reflect anorthogonal polarization component (X) , said means further comprising afolding prism operatively attached to said PBS to reflect X into saidfirst face in a spatially separate parallel plane.
 16. The opticalhybrid of claim 2, wherein said reflection surface is part at a foldingprism.
 17. The optical hybrid of claim 1, wherein said means forcombining comprises: a first optically transmitting element fixedlyattached to said second face; a second beam splitter fixedly attached tosaid first optically transmitting element; a first folding prismattached to said second beam splitter; a first gap between said firstbeam splitter and said first folding prism; a second opticallytransmitting element fixedly attached to said third face; a second beamsplitter fixedly attached to said second optically transmitting element;a second folding prism attached to said second beam splitter; a secondgap between said first beam splitter and said second folding prism. 18.The optical hybrid of claim 17, wherein at least one of said first beamsplitter, said second beam splitter or said third beam splitter isselected from a group consisting of an unpolarized beam splitter and abeam splitter designed for only one polarization.
 19. The optical hybridof claim 18, wherein said unpolarized beam splitter comprises a cube.20. The optical hybrid of claim 9, wherein said means for adjusting theoptical path length comprises means for adjusting the index ofretraction of a gas within at least one of said first cavity or saidsecond cavity.
 21. The optical hybrid of claim 17, wherein said meansfor adjusting the index of refraction N comprises at least one opticalflat positioned within at least one of said first gap or said secondgap.
 22. The optical hybrid of claim 17, further comprising means forseparating the polarizations of said S beam and said L beam prior totheir being operatively aligned into said first face.
 23. The opticalhybrid of claim 22, wherein said means for separating the polarizationscomprises a polarizing beam splitter (PBS) configured to transmit onepolarization component (Y) into said first face and reflect anorthogonal polarization component (X), said means further comprising afolding prism operatively attached to said PBS to reflect X into saidfirst face in a spatially separate parallel plane.
 24. A method,comprising: splitting a signal light beam (S) into a reflected signallight beam (RS) and a transmitted signal light beam (TS); splitting areference light beam (L) into a reflected light beam (RL) and atransmitted light beam (TL); and combining pairs of said RS TS, RL andTL to produce a plurality of combined output beams comprising an S+Lbeam, an S+jL beam, an S−L beam and an S−jL beam, wherein the phasedifference, between said S light beam and said L light beam in said S+jLbean, said S−L beam and said S−jL beam, with respect to the phasedifference between said S light beam and said L light beam in said S+Lbeam, is about 90 degrees, 180 degrees and 270 degrees respectively;wherein said steps of splitting the signal light beam S and splittingthe reference light beam L are carried out with a beam splittercomprising a first face, a second face, a third face, a fourth face andan internal reflecting/transmitting coating; the method furthercomprising the step of separating the polarizations of said S beam andsaid L beam prior to their being operatively aligned into said firstface.